High density alloys of tungsten have been found useful in military hardware as penetrators for piercing armor plate because of their high melting points, density and other physical properties. These alloys have been prepared by blending particles of tungsten with other metals, for example, nickel and iron, compacting the resulting mixture of metal particles and then sintering the compacted particle product at very high temperatures. The performance of these alloys, as penetrators, can be substantially improved by increasing their hardness, for example, by subjecting them to a swaging operation.
Among other factors, penetration performance is improved not only by increasing the hardness of the cylinder or column of these alloys but also by increasing their length to diameter ratio, which increases the kinetic energy per unit area of impact. It is well-known in the art that spin stabilized projectiles are limited for accurate flight to a length to diameter ratio up to about 4:1. It is rather easy to fabricate such a penetrator by sintering a cylindrical piece composed of tungsten alloy having a length to diameter ratio of about 5:1 and then subjecting the sintered piece to cold work to harden the same by placing it in a suitable die and then applying coaxial compressive forces at the ends thereof to obtain a work hardened penetrator having the desired length to diameter ratio of about 4:1.
The defeat of modern armor, however, requires penetrators having length to diameter ratios in ranges in excess of about 4:1, generally from about 15:1 to about 25:1, or even higher ratios are desired in an effort to maximize the above-mentioned kinetic energy per unit area of impact. Hardening such long rods or columns using coaxial compression is not satisfactory, because long columns tend to buckle under load and thus do not flow to fit the die cavity adequately. Other methods of cold working these alloys are well-known, for example, extrusion or rotary swaging, and each of these can be used for pieces having high length to diameter ratios. While each of these methods has the capability to introduce the desired amount of cold working overall, it has been found that working is not always adequately distributed throughout the cross-section thereof. Such variations can result in residual stress patterns in the worked component. If the residual stress is in the same direction as the principal loads during launch or impact, premature failure of the penetrator may occur. Conversely, if the residual stresses are in the opposite direction, performance may be enhanced.
Referring to the art, Dardell in U.S. Pat. No. 2,356,966 discloses a method of making shot comprising softening a bar by heating, cutting the bar at its softened point and pointing the adjacent ends of the cut pieces by hammering while the shot is rotated, whereby two pointed shots are formed.
Sczerzenie et al., in U.S. Pat. No. 3,888,636 are interested in preparing an armor piercing penetrator comprising about 97 weight percent tungsten, 1.5 weight percent each of nickel and iron and to the process for making it. The sintered product is slow cooled and then quenched to harden it.
Northcutt, Jr., et al., in U.S. Pat. No. 3,979,234 disclose a process for making penetrators from tungsten, nickel and iron alloy which includes sintering the compacted powders, vacuum annealing the sintered product, and then cold working to achieve a high uniform hardness. The patentees state that swaging is the preferred form of cold working and suggest that other cold working processes can be used. No other cold working processes are specified, however.
In U.S. Pat. No. 4,441,237, Kim et al. disclose penetrators made from a continuous rod of a metal matrix composite material which involves heating sections of the rod by induction heating then twisting the softened sections to form confronting nose sections of two projectiles. Different nose shapes are obtained by varying the length of the heat-softened section. The patentees state that the twisting of the softened region causes the fibers in the nose to cross, thereby forming a harder nose than the main body of the projectile due to increased volume percentage reinforcement in the nose.
Mullendore et al. in U.S. Pat. No. 4,458,599 disclose a tungsten penetrator and a process for making the same in which the sintered bar is elongated by swaging, thereby reducing the cross sectional area of the bar, machining it to the desired shape and then annealing to obtain a bar of desired hardness.
None of the above references, taken alone or in combination, teaches or suggests working a cylinder or column of tungsten alloy by torquing the rod beyond the yield point to produce a penetrator which is hard at the surface, tough in the center to resist bending, and with a hardness gradient such that the surface hardness is materially harder than the center or the core thereof.